Is America losing its faith in religion? The answer would seem to be yes, judging by polls and news stories lately. Gallup announced in May that 77% of Americans believe that religion is losing its "influence on American life." Reporting online about the Gallup results, The Blaze said the poll "suggests that America's slide toward secularism continues to gain steam."In March at the Faith Angle Forum in South Beach, Fla., a paper by the Pew Forum on Religion & Public Life was presented bearing the title "The Decline of Institutional Religion." The presentation followed up on Pew research that gained wide publicity last fall indicating that the fastest-growing "religious" group in America is made up of those who say they have no religion.According to Pew, 8% of Americans in 1990 gave their religious preference as "none." By 2007, that response had nearly doubled to 15%, and in 2012 the "no religion" response had climbed to 20%. Earlier this year, an analysis of the General Social Survey by the National Opinion Research Center at the University of Chicago tracked a similar trend, also citing the 20% no-religion response.Many interpret the numbers to mean that America is heading down the secular road. In a survey published this month by the Pew Research Center, 48% of Americans say the growing number of "people who are not religious" is a bad thing for American society (and only 11% say it is a good thing).But I disagree with the notion that the U.S. is heading toward becoming as unchurched as much of Europe. One reason is that saying you have "no religion" is not the same as disbelieving in God. Many people who say they have no religion are simply saying they have no official religious affiliation. They may actually have strong personal beliefs. The increase in the "no religion" group may also be an illusion caused by the rising nonresponse rate to survey studies.Consider: The proportion of Americans who claim to be atheists has not increased even slightly since Gallup first asked about belief in God in 1944. Back then, 4% said they did not believe in God, and 3% or 4% give that answer today.Most of those Americans who are reported as having no religion are not unreligious but only unaffiliated, and some of them even attend church. They do not belong to any specific denomination, but probably most of them would agree that they are Christians, had they been directly asked that question.A far more important indicator, as many recent studies—including the Baylor National Religion Surveys—have found, is that those who say they have no religion are surprisingly religious. Most say they pray, and a third even report having had a religious experience. Half of these respondents who would be considered by survey takers to have "no religion" believe in angels.So even if the proportion of Americans with no professed religion is rising, that does not translate into an increase in irreligiousness. But it may well be that the proportion of nonreligious Americans is not even increasing, and remains far smaller than recent surveys reveal.When I was a young sociologist at Berkeley's Survey Research Center, it was assumed that any survey that failed to interview at least 85% of those originally drawn into the sample was not to be trusted. Those who refused to take part in the survey or could not be reached were known to be different from those who did take part. Consequently, studies were expected to report their completion rates.Today, even the most reputable studies seldom reach more than a third of those initially selected to be surveyed and, probably for that reason, completion rates are now rarely reported. The Pew Forum researchers are to be commended for reporting their actual completion rates, which by 2012 had fallen to 9%.Given all of this, only one thing is really certain: Those who take part in any survey are not a random selection of the population. They also tend to be less educated and less affluent. Contrary to the common wisdom, research has long demonstrated that this demographic group is the one least likely to belong to a church.As the less-affluent and less-educated have made up a bigger share of those surveyed, so has the number of those who report having no religion. That would help explain why, during this whole era of supposed decline, Baylor surveys find that the overall rate of membership in local religious congregations has remained stable at about 70%. Hard to write a headline about the lack of change. Sometimes, though, no news really is good news.Mr. Stark, co-director of the Institute for Studies of Religion at Baylor University, is the author of "The Triumph of Christianity" (HarperCollins, 2012).A version of this article appeared July 4, 2013, on page A9 in the U.S. edition of The Wall Street Journal, with the headline: The Myth of Unreligious America.